Abstract
Exfoliation of two-dimensional boron nitride nanosheets (BNNSs) from parent bulk material has been receiving intensive attention because of its fascinating physical properties. Liquid exfoliation is a simple, scalable approach to produce single-layer or few-layer BNNS. In this paper, water/propanol co-solvent exfoliation of bulk boron nitride under the assistance of sonication was investigated in detail. Special attention was paid on the effect of raw bulk boron nitride size and co-solvent composition. The results show that sonication of small-size hexagonal boron nitride tends to generate large nanosheets, due to a predominant solvent wedge effect. In addition, it is found that the composition of water/propanol co-solvent has an important effect on exfoliation efficiency. Interestingly, although two isomers of 1-propanol (NPA) and 2-propanol (IPA) have the same molecular weight and similar surface tension, their aqueous solutions allow the formation of boron nitride nanosheets dispersion with markedly different concentrations. It is proposed that due to their spatial configuration difference, NPA with its longer molecular chain and fewer hydrophobic methyl group tends to form dynamic water-NPA clusters with larger size than water-IPA clusters. The hydrodynamic radius of the co-solvent “clusters” was calculated to be 0.72 nm for water/NPA system and 0.44 nm for water/IPA system at their maximum, respectively. Their size changes, represented by two curves, indicate a strong “cluster size” effect on exfoliation efficiency. Our work provides an insight into co-solvent exfoliation of hexagonal boron nitride and emphasizes the importance of co-solvent cluster size in exfoliation efficiency.
Highlights
Since the first successful isolation of monolayered graphene in 2004, two-dimensional nanomaterials have witnessed an explosive growth in research interest [1,2] due to their exceptional properties for many promising applications
In order to clarify the size effect of hexagonal boron nitride (h-BN) on the resultant boron nitride nanosheets (BNNSs), h-BN raw materials with a size of 2 μm and 30 μm are chosen for comparison
The results indicate that a larger h-BN does not necessarily produce larger-sized BNNSs
Summary
Since the first successful isolation of monolayered graphene in 2004, two-dimensional nanomaterials have witnessed an explosive growth in research interest [1,2] due to their exceptional properties for many promising applications. Ultrasonic-assisted exfoliation is a process in which the van der Waals interactions between the atomic layers of bulk boron nitride are overcome by ultrasonic energy, and the detached individual nanosheets are subsequently stabilized by solvent molecules. With the ultrasonic vibration [17] as a driving force, the strong affinity of solvent molecules on the surface of h-BN is necessary In this case, a minimization of their interfacial energy [16,18,19] is essential to achieve efficient exfoliation. Among the various mixtures of solvents, co-solvents consisting with water and organic solvents are considered a promising route for exfoliation of h-BN because of their reduced cost, improved safety and easy scalability [28,29], as the number of solvents and possible solvent mixtures is numerous, it is important to establish a methodology to choose appropriate solvents/co-solvents and probe the related liquid exfoliation mechanisms
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